Received 4 July 2020; revised 31 July 2020 and 22 August 2020; accepted 22 August 2020. Date of publication 25 August 2020;date of current version 12 November 2020. The review of this paper was arranged by Associate Editor Yunlong Zi.

Digital Object Identifier 10.1109/OJNANO.2020.3019425

Stretchable Triboelectric Nanogenerators forEnergy Harvesting and Motion Monitoring

JIAHUI HE , YIMING LIU, DENGFENG LI, KUANMING YAO, ZHAN GAO, AND XINGE YUDepartment of Biomedical Engineering, City University of Hong Kong, Hong Kong 999077, China

CORRESPONDING AUTHOR: XINGE YU (e-mail: xingeyu@cityu.edu.hk)

This work was supported by City University of Hong Kong under Grants 9610423 and 9667199, Research Grants Council of the Hong Kong SpecialAdministrative Region under Grant 21210820, and Science and Technology of Sichuan Province under Grant 2020YFH0181. Jiahui He and

Yiming Li contributed equally to this article.This article has supplementary downloadable material available at https://ieeexplore.ieee.org, provided by the authors.

ABSTRACT Motion monitoring by flexible strain or pressure sensors have been under spotlight in the fieldof wearable electronics. Based on triboelectric effect, generated energy from body contact and compressionduring daily movement can be used for both reflecting motion status and energy recollection. Here, we reporta stretchable pressure sensor based on triboelectric effect and dots-distributed metallic electrodes, adoptingcontact-separation mode. The dots-distributed electrode based triboelectric nanogenerator (D-TENG) couldbe easily integrated with body and cloth, such as on the skin and under foot, to sense a broad range of activityrelated strain information. The D-TENGs enable accurate detecting a broad range pressure from ∼5 kPa to∼50 kPa with open circuit voltage variation from several volts to tens of volts, and thus allow monitoringbody daily actives such as joints’ bending, walking and running. These devices maintain stable and high-levelsignal outputs even after thousands cycles of measurement, proving the good stability. Simultaneously, themechanical energy produced by our body motions could also be recollected by the D-TENG sensor for energyharvesting. Under a constant tapping by finger (39.59 kPa), the induced voltage is sufficient to light up 15LEDs. The stretchable D-TENG sensor indicates its great potential in motion monitoring and mechanicalenergy harvesting.

INDEX TERMS Energy harvesting, motion monitoring, stretchable electronics, strain sensors, triboelectricnanogenerators.

I. INTRODUCTIONThe great applications of wearable electronics, such as health-care [1]–[6], human-machine interface [7], [8] and motionrecognition [9]–[11], have attracted much attention over thepast decades. Compared to the traditional electronics basedon rigid and brittle Si platform, flexible electronics exhibitgreat deformability and excellent electrical properties [12]. Inrecent years, intrinsically stretchable/flexible materials [13]–[17] and fancy structural mechanics designs, i.e. serpentine[18], [19], island-bridge [19], [20], and nano-mesh [21], [22],have been adopted to develop various kinds of flexible devicesfor monitoring humidity [23], temperature [24], [25], pressureand human motion [26]–[29]. To fully realized the advantage,bulky parts such as batteries are needed to be replaced by thin,soft self-power components. To date, self-power technologiesbased on thermoelectric [30], [31], optical effects [32], [33]

piezoelectric[34]–[36], and triboelectric effects have been de-veloped [37]–[40]. Among them, triboelectric nanogenera-tors (TENGs) are good candidates as which can not onlyserve as energy harvesters but also act as self-powered sen-sors. TENGs can generate considerable electricity throughcontact-separation at the interface of two materials surfacewith different electrical natures [41], [42]. The human bodymotion induced contact-separations between skin and devicecould be used for motion monitoring as well as a platform formechanical energy harvesting [43], [44].

TENG-based sensors include single-electrode mode andcontact-separation mode, where single-electrode modeTENGs take advantage of simple structure and fabrication:only one triboelectric layer coupling with a conductive layer isrequired in a device. Given above, this laboratory has reportedfilmy, skin-integrated TENGs combing single-electrode mode

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FIGURE 1. Schematic diagram of a dots-distributed electrode-based triboelectric nanogenerator (D-TENG). (a) The schematic illustration of a D-TENG.(b) The stretchable electrode patterns. (c) Optical image of the D-TENG on the desk and (d) on human skin. (e) The D-TENG is tiny enough to be fitted intoa shoe. (f) Optical image of the D-TENG under stretching, twisting, and bending.

and structural mechanics design [28], [45]. Nevertheless, theperformance of single-electrode mode TENGs would varywhen contact with different objects, or work under differenthumidity [45], limiting their practicability in accuratepressure sensing and stable power supplying. In contrast, thecouple of functional triboelectric layers are encapsulated in acontact-separation mode TENGs, averting the above problem.Besides, metallic thin film, silver nanowire networks [27],[46], carbon nanotubes [47] and graphene [48] are the mostlyused electrodes for TENGs. Metallic thin film electrodes (goldor copper) present great and stable electrical conductivity thatis very suitable for a conductive layer as well as a properpassive triboelectric layer. Given the above, combing thestretchable layout of electrode with contact-separation modeTENG still need further study.

Here we report a contact-separation mode TENG sensorsbased on copper electrode with dots-distributed layout. Cop-per electrode and PDMS serve as positive and negative tribo-electric layers, respectively. The sensors present great sensi-tivity and stability under a broad range of strains/pressures.With the serpentine electrode patterns, the D-TENG exhibitgreat flexibility and durability under various mechanical sit-uations such as large-angle bending, twisting and stretching,maintaining stable signal output even after thousands cycles offinger beating with pressure of 17∼22 kPa. Moreover, bodymotion induced high output voltage of the D-TENG sensor

would be a good source for energy harvesting and self-powerwearable device. These contact-separation mode D-TENGsshow a great potential in energy harvesting and stable motionmonitoring.

II. MATERIALS AND METHODSThe D-TENG sensor adopt a symmetric structure with multi-ply layer stacking layouts, consisting of three layers of PDMSand two layers of copper thin film electrode. As shown inFig. 1(a), the top and bottom PDMS layers both serve as thesubstrates and encapsulation layers with same thickness. Toensure good adhesion and stretchability, PDMS (Sylgard 184,Dow Corning Corporation) was prepared by the proportionalcross-link agent and per-polymer of 1:20. The molds used forPDMS layers formation were realized by 3D printing. Theproportioned PDMS was poured into the molds, vacuumed for20 min to remove air bubbles, then cured in an oven at 40 °Cfor 5 h. Peeling off the PDMS from the molds formed twosubstrate PDMS layers. One layer is sized of 2 cm × 2 cm ×2 mm and the other contains a 2 cm × 2 cm × 2 mm substrateand four surrounded 1.1 mm-height ×1 mm-width walls. Thedevice can be adhered on the skin with either sides.

The middle 88- µm-thick PDMS film serves as negativetriboelectric layer. With a layer of stretchable 6- µm-thickcopper electrode, the negative triboelectric PDMS layer wasbonded on the top PDMS substrate. The stretchable copper

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FIGURE 2. The tensile test of D-TENG. (a) Optical images of the D-TENG under strain of 0%, 20% and 30%. (b) The stress-strain curve of D-TENG understrain from 0% to 30%.

electrode was prepared as follows. A piece of 6- µm-thickcopper foil was flattened by a cylinder on a glass substratewith a very thin PDMS layer. Then, AZ 4620 photoresist (AZElectronic Materials) was spin-coated on copper foil at 3000rpm for 30 s and soft baked at 115 °C for 5 min. With designedmask, the sample was exposed under a mask aligner (URE-2000/35AL deep UV, IOE, CAS) for 45 s. After developing inAZ 400K developer and post-bake for 5 min at 115 °C, the pat-terned copper foil was wet etched in FeCl3 solution. Thus, thestretchable copper electrode was realized. Then, after cleaningthe copper electrode by acetone, PDMS precursor (crosslinkagent: per-polymer = 1:10) was spin-coated on the electrodeat 1300 rpm for 30s to form an 88- µm-thick PDMS layeras the negative triboelectric layer. With un-crosslinked PDMSand subsequent curing process, the 88- µm PDMS layer withelectrode could be tightly attached on the top substrate andleaving the triboelectric layer facing down.

With the same method, we fabricated another stretchablecopper electrode on the bottom PDMS substrate with exposedelectrode facing up. Finally, the top and bottom substratewere adhered together using PDMS adhesive layer to ensurethe entire device encapsulated well. By then, the D-TENGdevice was completed. It works by the contact-separationmode between 88- µm PDMS negative triboelectric layer andbottom positive triboelectric layer (stretchable 6- µm copperelectrode). The output voltage and current can be derived fromboth the top and bottom electrodes.

The tensile test was carried out using a mechanical testingsystem (INSTRON 5942 Testing System) with an elongationspeed of 10 mm min−1 at room temperature. The sample wasa complete D-TENG with extra length for clamping. To testthe influence of the thickness of PDMS negative triboelectricon the D-TENG performance, we fabricated a series of PDMSthin layer with different thicknesses. The thickness was mea-sured by optical surface profiler (Veeco/Wyko NT9300). The

open-circuit voltage of the D-TENGs was measured by a DAQmultimeter (Keithley 6510) at a sampling rate of 60 kHz.The short-circuit current of the D-TENGs was calculated bymeasuring the voltage of a resistor connecting in series withthe D-TENG (through PL3516/P Powerlab 16/35, AD Instru-ments, at a sampling rate of 10 kHz ). The current couldbe calculated according to the voltage and resistance. TheD-TENG was operated and tested on a volunteer body withhis full and informed consent.

III. RESULTS AND DISCUSSIONTo develop a stable and stretchable device for motion monitor-ing, we combine TENG effect and mechanics design togetherto fabricate a contact-separation mode TENG sensor with twodots-distributed electrodes. Fig. 1(a) illustrates the schematicdiagram of the dots-distributed electrode-based triboelectricnanogenerator (D-TENG) sensor. To increase the stretchabil-ity of this sensor, we design the 6- µm-thick copper electrodeswith distributed dots connected by thin serpentines (Fig. 1(b)),that ensures a large voltage output as well as good stretchabil-ity. This sensor works through the contact-separation processbetween middle 88- µm-thick PDMS negative triboelectriclayer and bottom stretchable 6- µm copper electrode as pos-itive triboelectric layer, due to their corresponding electricalnegative and positive nature [49], [50]. The induced signalsincluding voltage and current are derived from both the topand bottom electrodes.

To realize the contact-separation process, the middle PDMSlayer with electrode is tightly attached on the top substratewith triboelectric layer facing down. Another stretchable cop-per electrode is on the bottom PDMS substrate with exposedelectrode facing up. The gap between them is about 1.1 mm.We realize this fixed gap by utilizing two 3D printed molds.After assembling the top and bottom parts, the device withsize of 2 cm × 2 cm × 4.5 mm is obtained, as shown in

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FIGURE 3. Electrical performances of the D-TENG sensor. (a) The open-circuit voltage and (b) short-circuit current by D-TENG as a function of PDMStriboelectric layer thickness under constant pressure of 15.03 kPa and frequency of 2 Hz. (c) The open-circuit voltage and (d) short-circuit current byD-TENG as a function of pressure under a frequency of 2 Hz. (e) The stability of D-TENG at constant pressure of 17∼22 kPa and frequency of 4 Hz for 1440cycles. (f) The transferred charge for capacitors with 0.1 µF after rectification by full-wave bridge rectifier, under a constant pressure of 45.23 kPa atfrequency of 5 Hz. (g) The open-circuit voltage and (h) short-circuit current by D-TENG as a function of frequency under constant pressure of 14.77 kPa.

Fig. 1(c). The entire device is encapsulated by PDMS sub-strate to prevent the corrosion and oxidation of electrodes,caused by sweat, oxygen and vapour. The minimum width lineof electrode pattern is 400 µm, the weight of an as-fabricatedD-TENG sensor is 1.54 g. Such light device is very portable.Actually, both sides of the sensor can be easily and nonirri-tatingly attached on human skin by the low modulus PDMS(1:20) interfaces (Fig. 1(d)). For further motion monitoringand energy harvesting, we also integrated the D-TENG sensorinto a shoe, as demonstrated in Fig. 1(e).

Fig. 1(f) presents that the D-TENG sensor owns good flexi-bility and stretchability. PDMS has already been commonlyused in flexible electronics for its good physical/chemical

stability and flexibility [51]. The substrates with soft 1:20PDMS allows the high degree deformation. The design ofdots-distributed electrode associate with arrayed separatedsquare electrodes connecting by serpentines traces, offeringthe Cu electrode good stretchability and flexibility. The sep-arated square electrodes are designed to increase the contactarea of the triboelectric surfaces, which will enhance the elec-trostatic induction between triboelectric layer and electrode,and magnify the triboelectric performance [52]. As shown inFig. 1(f), the electrodes and device still remain robust evenunder 25% strain, twisting and 180 degrees bending. In addi-tion, a tensile test was carried out to furtherly exhibit its greatstretchability, as shown in Fig. 2. Under an increasing uniaxial

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FIGURE 4. D-TENG sensor on the skin for pressure sensing and energy harvesting. (a) Optical images showing four different external loads, namedtouching, poking, tapping, and hitting, by a finger on the D-TENG. The sensor is attached on the skin (b) The open-circuit voltage and short-circuit currentby D-TENG under different loads. (c) 15 LEDs were lighted up by the D-TENG under fingers beating.

strain from 0% to 30%, the original length (20 mm) of thedevice was lengthened to 26 mm, however, there was no anyvisible crack on the entire structure (Fig. 2(a)); meanwhile, thestress-strain curve (Fig. 2(b)) presents quite low stress evenunder high-level strain, indicating its successful mechanicsdesign and good portability.

We select the 1:10 PDMS to fabricate the triboelectric layerof D-TENG. Its stiffness can achieve good triboelectric per-formances with proper stretchability. Meanwhile, the surfaceis less sticking to ensure the good separation ability duringcontact-separation process. In a contact-separation processdriven by constant pressing, the electrons would inject fromthe copper electrode to the PDMS; when the D-TENG isreleased, the PDMS surface separates away from the buttoncopper electrode, leaving a potential difference between thecouple of electrodes, and the induced electrons would flowfrom negative side to positive side, as shown in Fig. 1. Basedon above working principle, the D-TENG could transfer me-chanical energy into electricity. To study the influence oftriboelectric layer’s thickness on the device electrical perfor-mance, we fabricated the devices with different thick middlePDMS layers. By adjusting the spin-coating rpm, the middlePDMS layer with thickness of 1.02mm, 510 µm, 340 µm,170 µm, 102 µm and 88 µm were fabricated. The thick-ness was measured by optical surface profiler (Veeco/WykoNT9300). Fig. 3(a) and b exhibits the open-circuit voltage andshort-circuit current as functions of a series of PDMS thick-ness. Under external pressure of 15.03 kPa at 2 Hz, the output

signal of D-TENG increased obviously with the decrease ofPDMS layer thickness. For thickness of 1.02 mm, open-circuitvoltage and short-circuit current are 12.61 ± 0.35 V and 3.75± 0.32 µA, respectively. When the thickness decreases to88 µm, the corresponding peak values of open-circuit voltageand short-circuit current are up to 39.7 ± 1.17 V and of 15.2± 0.44 µA. The result indicates that the thinner triboelectriclayer would perform better electrical signals by enhancing theelectrostatic induction between the electrode and triboelectriclayer. However, considering the triboelectric performance anddevice stability, the 88 µm in this work is an optimized value.[53], [54]. Therefore, the D-TENG with 88 µm PDMS tribo-electric layer was selected as a representative sample for thefollowing test.

Generally, larger pressure induces stronger triboelectric ef-fect. Fig. 3(c), d shows the open-circuit voltage and short-circuit current as functions of external pressure at a constantfrequency of 2 Hz. Obviously, both the voltage and currentincreased as the external pressure rises. Moreover, the outputsignal is also outstanding with 4.09 ± 0.24 V voltage and1.31 ± 0.12 µA current under only 5.9 kPa, which enablesthe sensing of small pressure. Then, the voltage and currentsignals increase to 88.7 ± 1.42 V and 34.9 ± 3.33 µA underlarge pressure of 50.98 kPa. In details, the measured value ofopen-circuit voltage was quite considerable and highly sen-sitive among the applied pressure, the sensitivity were 3.97± 0.17 mV/Pa (< 17.03 kPa) and 1.13 ± 0.11 mV/Pa (>17.03 kPa) respectively, for which the device can be used

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FIGURE 5. D-TENG sensors for motion monitoring. Optical images and the corresponding output signals for different body motions. (a) finger jointbending, (b) walking, (c) running, (d) jumping, and (e) climbing.

for pressure sensing without any additional amplifiers. Thewaveforms of the signal under the 50.98 kPa pressure areshown in the right side of Fig. 3(c) and d. The signals am-plitude keeps uniform under such large pressure, indicatingthat the D-TENG remains pretty good performances and thetriboelectric layers also achieve good contact and separation.

Besides, to test the stability of the D-TENG in practicalapplication, a constant pressure about 17 ∼ 22 kPa at a fre-quency of 4 Hz was applied on the device for 6min by humanfinger beating. As shown in the Fig. 3(e), the output signalgenerated from the D-TENG was quite stable during 1440cycles, about 16 ∼ 27 V, which proves the ruggedness of thedevice structure.

In addition, the charging performance test of the device for0.1 µF capacitance was carried out in a rectified circuit, asshown in Fig. S2. When a capacitor was connected to thecircuit, the charge generated from the device transferred tothe capacitor. As shown in Fig. 3(f), by constantly tapping thedevice with a pressure of 45.13 ± 1.42 kPa at frequency of5 Hz, the capacitor reached up to 100 mC within 20 s.

The electrical performances of D-TENG under constantpressure of 14.77 kPa at frequencies of 1 Hz, 2 Hz, 3 Hz,4 Hz, 5 Hz and 6 Hz were also demonstrated in Figs. 3(g)and (h). Even though the frequency varied, both the open-circuit voltage and short-circuit current changes a little. Thespecific values are 35.37 ± 2.9 V, 40.04 ± 1.04 V, 40.53 ±

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0.98 V, 43.92 ± 1.61 V, 51.68 ± 2.03 V, 51.50 ± 2.66 Vand 14.65 ± 0.74 µA, 15.21 ± 0.44 µA, 16.79 ± 1.52 µA,17.76 ± 2.08 µA, 20.88 ± 1.68 µA, 24.52 ± 0.9 µA. Theslightly signal increase may result from the increasing in-ductive impedance of DAQ multimeter with the increasingfrequency of AC signal. By the way, the signal interval isalways corresponding to the frequency of the applied force,indicating that the D-TENG owns good responsiveness evenunder high-frequency external pressure. Overall, the resultsunder high external force, high-frequency beating or long-time operating shows good response and stability of D-TENG,exhibiting its realizable potential in pressure sensor, bodymotions monitoring and wearable energy harvesting in dailylife.

Integrated on the skin, the D-TENG sensor can realize var-ious pressure sensing as a wearable device. We applied fourkinds of different external pressure loads, named touching,poking, tapping, and hitting, on the sensor to estimate itspractical performance (Fig. 4(a)). Attached on a volunteer’swrist, the sensor was touched, poked, tapped and hit by humanfingers at the frequency of 2 Hz. The responding pressures are4.5 ± 1.58 kPa, 15.08 kPa ± 2.12 kPa, 39.59kPa ± 1.87 kPaand 53.8 ± 2.91 kPa, respectively. The Fig. 4(b) shows thatvalues of the output signals increase with the loaded pressurerising. For general slight touching, the open-circuit voltageand short-circuit current reached 3.47 ± 0.32 V and 1.32 ±0.23 µA, respectively, which proves that the pressure sensoris quite sensitive for the pressure sensing. When heavily hit-ting the sensor, the corresponding values boosted up to 88.75± 1.42 V and 31.91 ± 3.96 µA. Therefore, the D-TENGsensor shows a very wide range of pressure sensing. Besidespressure sensing, the generated electrical signal could also berecollected for energy harvesting. Fig. 4(c) demonstrates that15 LEDs were lighted up by a D-TENG sensor in a rectifiedcircuit (Fig. S2) under the tapping of a finger (∼37 kPa, MovieS1). Therefore, during pressure sensing, the D-TENG sensoris also playing a role in energy conversion and storage. Theoutput voltages would also be a good source for the self-powerwearable devices.

Physical motion of the human body commonly includesjoints movement and overall body movement. To furtherdemonstrate the potential application of D-TENG in motionmonitoring, we integrated the sensors on the finger joints orinto the shoes for pressure sensing, as shown in Fig. 5. Forexample, when the finger bends, the mounted D-TENG willgenerates the contact-separation process (Fig. 5(a)). For 120degree bending at 2 Hz, the open-circuit voltage and short-circuit current reached 4.5 ± 0.12 V and 5.39 ± 0.79 µA,respectively. On the finger, the D-TENG sensor presents goodflexibility.

For actual excise, the overall body often continues to riseand fall. When body falls, the potential energy will do a lotof work, which is much greater than the kinetic energy ofthe joint. As shown in Figs. 5(b-e), the D-TENG sensor wasmounted on the insole. During a volunteer’s motion of walk-ing (∼16.7 kPa), running (∼50.8 kPa), jumping (∼58.4 kPa)

and climbing (∼18.4 kPa), the corresponding voltage andcurrent were 42.09 ± 5.62 V, 90.14 ± 5.16 V, 93.12 ± 8.04 V,49.42 ± 3.36 V and 17.87 ± 3.51µA, 29.7 ± 5.49 µA, 30.47± 1.71 µA, 20.43 ± 1.78 µA. Walking and climbing showscomparable signal output. Similarly, the values of output volt-ages for running and jumping is similar. Overall, the values ofoutput signal are related to the potential energy of the motions.The results indicate the TENG devices can be used for variouskind of activity monitoring, such as sensing of joint motions,steps, postures, and many others.

More importantly, the detected output signal is considerableenough for directly recording without any amplification. Thegood mechanical performance, stable and outstanding electri-cal output is quite suitable for continuous motion monitoring.Through the above practical application testing, the excellentpotentials of D-TENG in motion monitoring and power charg-ing application are well exhibited.

IV. CONCLUSIONIn conclusion, we developed a soft, stretchable triboelectricnanogenerator working by contact-separation mode for en-ergy harvesting and motion monitoring. The mechanical de-sign of dots-distributed electrode endows the sensor goodstretchability. By simple fabrication process, the acquired sen-sors could be integrated on the skin for pressure sensing withstriking signal output. The sensing covers a wide pressurerange from slight touching to heavy hitting. Moreover, after1440 cycles, the sensor still shows stable high signal output.For general excise, the sensor mounted on the shoes couldrecord the motion including walking, running, jumping, andclimbing. Along with the motion monitoring, the generatedhigh voltages can also light up 15 LEDs easily. This D-TENGsensor, as a wearable device, shows its outstanding perfor-mance in energy harvesting and motion monitoring, which in-dicates its quite realizable prospects in the broad applicationsof wearable devices.

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